High intensity discharge arc lamps commonly operate in three modes:
(1) a cold or starting mode, comprising a glow breakdown state followed by a glow to arc transition state, in which a relatively high value of a-c or d-c type starting voltage is applied across the lamps electrodes to first place the gases of the lamp into a suitable ionized condition for striking or initiating a glow state, which is followed by a delay time period of up to approximately a minute so as to allow a transition into an arc condition between the electrodes of the lamp;
(2) an operating mode, in which the arc discharge of the lamp generates a desired light output and a relatively low or moderate voltage occurs across the electrodes of the lamp in response to a suitable arc discharge current as established by the ballast or operating circuit related to the lamp; and
(3) a hot restart mode, in which the arc discharge of the lamp fails or extinguishes for some reason such as a momentary interruption of the current supplied to the lamp. If the arc condition extinguishes, the lamp is permitted to cool for a period of up to a minute or more before the arc condition can be restarted by the relatively high starting voltage.
An example of a prior art circuit for operating gas discharge lamps is disclosed in U.S. Pat. No. 4,340,843, issued July 20, 1982 to Thomas E. Anderson and assigned to the present assignee. The Anderson patent discloses a circuit for starting and operating a compact high intensity arc discharge lamp which circuit includes a keep alive feature to prevent the arc condition of the lamp from extinguishing during undesired power fluctuations. The keep alive feature is provided by a related circuit for activating a pulse generating circuit connected to the lamp whenever the arc current within the lamp fell below a predetermined value and also inactivating the pulse generating circuit whenever the arc current rose above a second predetermined value.